In hip replacement surgery, the acetabular cup implant area is prepared prior to positioning of the implant, in order to ensure a perfect fit thereof. Typically this involves removal of deteriorated or diseased bone surface, but also carving the opening to match the size of the implant to be fitted. This is performed using an acetabular cup reamer. This is typically a tool with a handle and a rotating head. Reamer heads of increasingly larger size (optionally provided as removable heads of one device) are used to enlarge the acetabulum during the reaming procedure.
Typically reaming starts at a diameter of about 6-8 mm less than the size of the implant, and sequential reaming is done with increments in 1-2 mm. However, the thickness of the acetabular wall should be taken into account. When thinning is present, the reamer should be pulled toward the opposite wall to avoid the generation of a rim defect. In order to ensure correct orientation of the opening, the position of the reamer should mimic the final position of the acetabular component. The finished acetabulum should have the cotyloid fossa removed and the acetabular rim snug against the finishing reamer.
An exact fit of the implant in the reamed acetabulum is critical as misalignment of the implant can lead to increased wear rate for the implant or improper seating of the implant leading to restricted leg movement and even hip dislocation.
Accurate reaming is however hampered by three important factors: a) the variability of patient-specific features, b) the limitation of the manipulation of the reamer by the surgical window, c) the limited visual access of the target area and d) the fact that precise manipulation of a rotary tool is difficult.
US 2002099288 describes a system which uses ultrasound imaging for guiding the placement and orientation of the implant relative to the anatomical landmarks surrounding the acetabulum such as the greater sciatic notch, the spine of the ischium, gemellus superior, obturator foramen, the anterior inferior spine and others.
US 2008009874 describes a reamer with a shaft onto which a one-piece guide is attached which includes a locator member which extends to a first portion of the bone providing a reference point relative to the reamer while the reamer engages a second portion of the bone during cutting. One reference point, however, does not suffice to restrict all degrees of freedom.
There is a need for reamers which can ensure precise and accurate reaming, to ensure optimal fitting of the acetabular cup implant.